Optical line terminal arrangement, apparatus and methods

ABSTRACT

A wavelength division multiplexed optical communication system includes a plurality of optical line terminals which may be part of separate in service networks, each having a line interface and an all-optical pass-through interface including a plurality of pass-through optical ports, and each also including a plurality of local optical ports which are connectable to client equipment and an optical multiplexer/demultiplexer for multiplexing/demultiplexing optical wavelengths. The optical multiplexer/demultiplexer may include one or more stages for inputting/outputting individual wavelengths or bands of a predetermined number of wavelengths, or a combination of bands and individual wavelengths. At least one of the pass-through optical ports of an optical line terminal of one network may be connected to at least one of the pass-through optical ports of an optical line terminal of another network to form an optical path from the line interface of the optical line terminal of the one network to the line interface of the optical line terminal of the another network to form a merged network. The use of such optical line terminals allows the upgrading and merging of the separate networks while in service.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 60/112,510, filed Dec. 14, 1998.

FIELD OF THE INVENTION

[0002] The invention is in the field of optical telecommunications, and more particularly, pertains to upgrading an in-service wavelength division multiplexed (WDM) optical communication system including a pair of optical line terminals (OLTs) that reside in the same office and are part of separate WDM networks to form an all optical pass-through from the line side of one OLT of the pair to the line side of the other OLT of the pair.

BACKGROUND OF THE INVENTION

[0003] Wavelength division multiplexing (WDM) is an approach for increasing the capacity of existing fiber optic networks. A WDM system employs plural optical signal channels, each channel being assigned a particular channel wavelength. In a WDM system optical signal channels are generated, multiplexed to form an optical signal comprised of the individual optical signal channels, transmitted over a single waveguide, and demultiplexed such that each channel wavelength is individually routed to a designated receiver.

SUMMARY OF THE INVENTION

[0004] In typical wavelength division multiplexing systems all wavelengths are constrained to pass through from a source optical node to a predetermined sink optical node.

[0005] In view of the above it is an aspect of the invention to selectively pass-through, add or drop individual wavelengths at selected optical nodes.

[0006] It is another aspect of the invention to utilize optical line terminals having all-optical pass-through interfaces that provide for continued transmission of optical signals without any intervening electro-optical conversion, and to connect two optical line terminals back-to-back at their respective pass-through interfaces to provide an optical path from the line side interface of the first optical line terminal to the line side interface of the second optical line terminal.

[0007] It is yet another aspect of the invention to utilize optical line terminals having a multiplexer/demultiplexer including one or more stages for inputting/outputting individual wavelengths or bands of a predetermined number of wavelengths, or a combination of bands and individual wavelengths.

[0008] It is a further aspect of the invention to utilize the optical line terminals to support complex mesh network structures while permitting growth of an in-service network without disrupting network service.

[0009] It is yet a further aspect of the invention to provide a wavelength division multiplexed optical communication system including a plurality of optical line terminals, each having a line interface and an all-optical pass-through interface including a plurality of pass-through optical ports and each also including a plurality of local optical ports and an optical multiplexer/demultiplexer for multiplexing/demultiplexing transmitted/received wavelengths. The optical multiplexer/demultiplexer may include one or more stages for inputting/outputting individual wavelengths or bands of a predetermined number of wavelengths, or a combination of bands and individual wavelengths, with at least one of the pass-through optical ports of one of the optical line terminals being connected to at least one of the pass-through optical ports of another optical line terminal to form an optical path from the line side interface of the one of the optical line terminals to the line side interface of the another optical line terminal.

[0010] These and other aspects and advantages of the invention will be apparent to those of skill in the art from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a block diagram of an optical line terminal;

[0012]FIG. 2 is a flow chart of the control steps executed by the controller 10 of FIG. 1;

[0013]FIG. 3 is a block diagram of an optical line terminal having a two-stage multiplexer/demultiplexer;

[0014]FIG. 4 is a schematic diagram representative of the optical line terminal of FIG. 1 or FIG. 3;

[0015]FIG. 5 is a schematic diagram of two optical line terminals such as in FIG. 4 being connected back-to-back;

[0016]FIG. 6 is a diagram illustrating how at least two separate point-to-point WDM systems can be upgraded while in-service to form a merged point-to-point WDM system;

[0017]FIG. 7 is a diagram illustrating how at least two separate network WDM systems can be upgraded while in-service to form a merged network WDM system; and

[0018]FIG. 8 illustrates a mesh connection between a plurality of optical line terminals.

DETAILED DESCRIPTION

[0019]FIG. 1 is a block diagram of an optical line terminal (OLT) 2 which is the basic element of the present embodiment. The OLT 2 has an input/output line interface 4 which is connected to an external fiber facility and transmits/receives an optical signal having N optical wavelengths, for example 32 wavelengths, on a single optical fiber which is multiplexed/demultiplexed by a multiplexer/demultiplexer 6, which outputs demultiplexed wavelengths λ1-λN on individual optical fibers. The respective wavelengths λ1-λN are sent either to a peer OLT via a pass-through port or to client equipment via a transponder and a local port. The client equipment includes SONET equipment, add/drop multiplexers, cross-connect switches, internet protocol (IP) routers, asynchronous transfer mode switches (ATM) and the like.

[0020] As employed herein an optical signal is generally intended to encompass wavelengths in the range of approximately 300 nanometers to approximately 2000 nanometers (UV to far IR). This range of wavelengths can be accommodated by the preferred type of optical conductor (a fiber optic), which typically operates in the range of approximately 800 nanometers to approximately 1600 nanometers.

[0021] Consider λ1 which is provided to a 1×2 switch 8 which is controlled by a control signal, having at least N states, from a controller 10. The controller 10 responds to a command, from a management system (not shown), at a terminal 12 to provide the control signal at a terminal 14 and then to control terminal 16 of switch 8 to position the switch 8 in a first or second position. When in the first position, λ1 is provided to a transponder 18 which transmits λ1 to a client apparatus 20 via a local port 19. When in the second position λ1 is provided to a pass-through port 22 to a corresponding pass-through port in a peer OLT 24. The control signal is also provided to output terminal 15, and then to control terminal 16 of a corresponding switch 8 in peer OLT 24 to route λ1 to the corresponding multiplexer/demultiplexer 6. If it is desired to send λ1 to both client apparatus 20 and peer OLT 24, an optical splitter can be used in place of the switch 8.

[0022] Switch 26 selects λ1 coming from the opposite direction in response to a control signal at terminal 28 from controller 10 to position switch 26 in a first or second position. When in the first position, λ1 is received from client 20 via local port 19 and transponder 18, and when in the second position λ1 is received from peer OLT 24 via pass-through port 22, and then is provided to multiplexer/demultiplex 6 to be multiplexed with the other received wavelengths λ2-λN.

[0023] A wavelength can be directly passed-through to a peer OLT rather than being sent to a client apparatus. For example, λ2 is directly sent to, and received from, peer OLT 30 via pass-through port 32.

[0024] A 1×N switch can be used to send/receive a wavelength to/from one of N−1 peer OLTs or a client apparatus. For example, 1×N switch 34 under control of a control signal, having at least N states, provided to terminal 36 from controller 10 sends λ3 to either peer OLT 38 via pass-through port 40, or peer OLT 42 via pass-through port 44, or peer OLT 46 via pass-through port 48 or client apparatus 50 via transponder 52 and local port 53. Reception of λ3 in the opposite direction is controlled by N×1 switch 54 under control of a control signal provided to terminal 56 from controller 10, and than is provided to multiplexer/demultiplexer 6 to be multiplexed with the other received wavelengths.

[0025] As discussed above, a wavelength can be passed-through to a peer OLT via a pass-through port or can be optically switched to a client apparatus via a local port. λN is shown as being directly passed through to, or received from, peer OLT 60 via pass-through port 62.

[0026]FIG. 2 is a flow chart of the steps performed by the controller 10 of FIG. 1 to control the 1×2 switches 8 and 26, and the 1×N switches 34 and 54 to route the respective wavelengths λ1-λN.

[0027] In step S101 the controller 10 waits for a command from a management system such as a computer (not shown). At step S102 a determination is made as to whether or not the command is a switch control signal to either pass-through the wavelength via a pass-through port to a peer OLT or drop/add the wavelength locally at/from a client apparatus via a transponder and a local port. If the answer is no, the command is handled by another interface (not shown) at step S103. If the answer is yes, a signal is sent to switch A (for example switch 8 or 34) to move switch A to transmit position X (the selected position) at step S104, and at S105 a signal is sent to switch B (for example switch 26 or 54) to move switch B to receive position X (the selected position). At step 106 the control signal at terminal 15 of controller 10 is sent to the peer OLT to set its switches A and B in a corresponding manner. A loop-back is then made to step S101 to wait for the next command.

[0028] In the multiplexer/demultiplexer 6 of FIG. 1, 32 wavelengths on a single optical fiber received at line interface 4 are demultiplexed into 32 individual wavelengths λ1-λ32. However, according to another aspect of the invention the 32 wavelengths can be demultiplexed into bands, for example four bands of 8 wavelengths each, by a first multiplexer, and the resultant four bands can be processed by the OLT. According to another aspect of the invention at least one of the four bands of wavelengths can be demultiplexed by a second multiplexer/demultiplexer into its individual wave lengths such that the OLT can process the individual wavelengths of the at least one band and the remaining ones of the four bands.

[0029]FIG. 3 is a block diagram illustrating a modular OLT 200 having two stages of multiplexing/demultiplexing. The operation of the OLT 200 is described with respect to the demultiplexing operation; however, it is to be understood that the multiplexing is merely the reverse operation. It is to be noted that the 1×2 switches and 1×N switches shown in FIG. 1 are not included in FIG. 3 in order to simplify the drawing. However, it is to be understood that in practice such switches may be utilized in the practice of the invention. The OLT terminal 200 has an input/output line interface 202 which is connected to an external fiber facility and receives on a single optical fiber N, for example 32, wavelengths which are demultiplexed by a multiplexer/demultiplexer 204, which is situated on a first modular card, into M, for example 4, bands of 8 wavelengths each. The first band 206 (λ1-λ8) is demultiplexed into its 8 individual wavelengths by a multiplexer/demultiplexer 208, which is situated on a second modular card, with each such wavelength being provided to a pass-through port (P) or a local port (L) via transponder (T). Each of the pass-through ports (P) is situated on a different modular card, and each of the transponder (T) and its associated local port (P) are situated together on yet another modular card. Although direct connections are shown, as discussed above the respective wavelengths may be selectively switched to either of a local port (L) via transponder (T), or a pass-through port (P) as described with respect to FIG. 1.

[0030] The second band 210 (λ9-λ16) is provided directly to a pass-through port (P), and the third band 212 (λ17-λ24) is provided directly to a pass-through port (P).

[0031] The fourth band 214 (λ25-λ32) is demultiplexed into its 8 individual wavelengths by a multiplexer/demultiplexer 216, which is situated on a modular card 217, with each such wavelength being provided to a pass-through port (P) or a local port (L) via a transponder (T). Again, switching may be used to select a connection to either P or T.

[0032]FIG. 4 is a simplified schematic diagram representative of the OLT 2 shown in FIG. 1 or the OLT 200 of FIG. 3. However, it is to be noted that for simplicity only 16 wavelengths are utilized. The OLT 300 interfaces and operates in a bidirectional manner as discussed in detail with respect to FIGS. 1 and 3. The line interface 302 is adapted for wavelength division multiplexed (WDM) optical communication signals of the highest relative order, in this example 16 wavelengths λ1-λ16, corresponding to the N optical wavelengths on a single optical fiber which are applied to input/output line interfaces 4 and 202 of OLT 2 (FIG. 1) and OLT 200 (FIG. 3), respectively. The pass-through interface connected to the lines WL 1-4, WL 5-8, WL 9-12 and WL 13-16 corresponds to the respective pass-through ports, and the local-interface connected to the lines labeled 16 local ports correspond to the local ports connected to the respective transponders, where wavelengths from or to client equipment are added or dropped.

[0033]FIG. 5 illustrates two OLTs 300A and 300B as shown in FIG. 4 connected in a back-to-back relationship by way of their respective all-optical pass-through interfaces. Thus, it is seen that the connection results in an optical add/drop multiplexer (OADM) functionality without requiring intermediate electro-optical conversion (OEO) of the communicated optical signals. As discussed above, the add/drop feature is achieved at the 16 local ports of each OLT, where channels (wavelengths) can be added or dropped by a manual configuration, or via add/drop switching, as controlled by switches 8 and 26 of FIG. 1, to achieve a switchable add/drop multiplexer.

[0034] The pass-through may be accomplished using single conductors and/or ribbon connectors that pass multiple individual channels (wavelengths) in one cable. The pass-through connections between OLTS 300A and 300B is preferably made using ribbon connectors/cables.

[0035]FIG. 6 illustrates three separate in-service WDM point-to-point optical communication systems A, B and C which are not initially interconnected. WDM system A includes optical nodes 400 and 402 which are optically connected via their respective line interfaces, with at least optical node 402 being an OLT. WDM system B includes optical nodes 404 and 406 which are optically connected via their respective line interfaces, with at least optical node 404 being an OLT. WDM system C includes optical nodes 408 and 410 which are optically connected via their respective line interfaces, with at least optical node 408 being an OLT.

[0036] As discussed above, the three separate WDM systems are not initially interconnected. However, any two of the three WDM systems, or all three of the WDM systems, may be interconnected by connecting respective OLTs of the separate WDM system back-to-back at respective pass-through ports as shown in FIG. 5, without disrupting service. For example, WDM system A may be connected to WDM system B by directly optically connecting pass-through ports of the OLT of node 402 to pass-through ports of the OLT of node 404 via optical fibers 416 and 418. WDM system A may also be connected to WDM system C by directly optically connecting pass-through optical ports of the OLT of node 402 to pass-through ports of the OLT of node 408 via optical fibers 420 and 422. Thus, an all optical path is provided from optical node 400 of WDM system A to optical node 406 of WDM system B, and likewise an all optical path is provided from optical node 400 of WDM system A to optical node 410 of WDM system C, resulting in a merger of WDM systems A, B and C without disrupting service. At the back-side of the respective optical nodes, lines with a box are indicative of local ports (L) to which client equipment is normally connected.

[0037]FIG. 7 illustrates three separate in-service WDM network optical communication systems D, E and F which are not initially interconnected. WDM system D includes optical nodes 500 and 502 which are optically connected via their respective line interfaces through an optical network 503, with at least optical node 502 being an OLT. WDM system E includes optical nodes 504 and 506 which are optically connected via their respective line interfaces through an optical network 507, with at least optical node 504 being an OLT. WDM system F includes optical nodes 508 and 510 which are optically connected via their respective line interfaces through an optical network 511, with at least optical node 508 being an OLT.

[0038] As discussed above, the three separate WDM optical networks are not initially interconnected. However, any two of the three WDM optical networks, or all three of the WDM optical networks may be interconnected by connecting respective OLTs of the separate WDM optical networks back-to-back at respective pass-through ports as shown in FIG. 5, without disrupting service. For example, WDM optical network D may be connected to WDM optical network E by directly optically connecting pass-through ports of the OLT of node 502 to pass-through ports of the OLT of node 504 via optical fibers 516 and 518. WDM system D may also be connected to WDM optical network F by directly optically connecting pass-through optical ports of the OLT of node 502 to pass-through ports of the OLT of node 508 via optical fibers 520 and 522. Thus, an all optical path is provided from optical node 500 of WDM optical network D to optical node 506 of WDM optical network E, and likewise an all optical path is provided from optical node 500 of WDM optical network D to optical node 510 of WDM optical network F, resulting in a merger of WDM network optical communication systems D, E and F without disrupting service. At the back-side of the respective optical nodes, lines with a box are indicative of local ports (L) to which client equipment is normally connected.

[0039]FIG. 8 illustrates how OLTs can be connected in more complex ways to achieve greater functionality, such as, for example, limited cross-connection capabilities. Specifically, OLT 600 and OLT 602 are connected back-to-back to form a first OADM, OLT 604 and OLT 606 are connected back-to-back to form a second OADM, OLT 600 and OLT 606 are connected back-to-back to form a third OADM and OLT 602 and OLT 604 are connected back-to-back to form a fourth OADM. OLT 600, OLT 602 and OLT 604 each have add/drop switching capability, whereas OLT 606 has no switching capability.

[0040] The arrangement shown in FIG. 7 illustrates how a group of OLTs in an office, which may be part of separate WDM networks, can be coupled to form different OADMs on an individual channel or per band basis. Wavelengths 1, 2, 3 and 4 (channels 1, 2, 3 and 4) are connected between pass-through optical ports of OLT 600 and OLT 602 via optical fiber 603 and are also connected between pass-through optical ports of OLT 604 and OLT 606 via optical fiber 607. Wavelengths 5, 6, 7 and 8 (channels 5, 6, 7 and 8) are connected between pass-through optical ports of OLT 600 and OLT 606 via optical fiber 608 and are also connected between pass-through optical ports of OLT 602 and OLT 604 via optical fiber 609. Wavelengths 9, 10, 11 and 12 (channels 9, 10, 11 and 12) can be separated into individual channels that are connected between local ports of the respective OLTs. For example, channel 9 is directly connected between a local port of OLT 600 and a local port of OLT 602 via optical fiber 610, and channel 10 is directly connected between a local port of OLT 600 and a local port of OLT 606 via optical fiber 612. To simplify the drawing, no connections are shown for wavelengths 11 and 12; however, they may be connected in a like manner. The local ports may also be connected to client equipment as discussed above. It is to be noted that the connection configuration of FIG. 8 does not constitute a plain patch-panel form of connectivity, insofar as it allows for switching of channels without manual reconfigurations.

[0041] In summary, the methods and apparatus of the present invention allow upgrading of a wavelength division multiplexed optical communication system including a pair of OLTs that reside in the same office or facility and are part of separate WDM networks (whether point-to-point links or more advanced networks) to form an OADM. Such upgrade is accomplished without service disruption to the network by appropriate connection of the OLTs through the pass-through interfaces.

[0042] Although certain embodiments of the invention have been described and illustrated herein, it will be readily apparent to those of ordinary skill in the art that a number of modifications and substitutions can be made to the preferred example methods and apparatus disclosed and described herein without departing from the true spirit and scope of the invention. 

What is claimed is:
 1. A wavelength division multiplexed optical communication system comprising: a first optical line terminal having a line interface and an all-optical pass-through interface; a second optical line terminal having a line interface and an all-optical pass-through interface; and connection means for connecting the all-optical pass-through interface of said first optical line terminal to the all-optical pass-through interface of said second optical line terminal to form an optical connection from the line side interface of said first optical line terminal to the line side interface of said second optical line terminal.
 2. The wavelength division multiplexed optical communication system of claim 1, wherein said connection means performs a manual connection.
 3. The wavelength division multiplexed optical system of claim 1, wherein said connection means includes add/drop switches in said first and second optical terminals.
 4. A wavelength division multiplexed optical communication system comprising: first through fourth optical nodes each having a line interface, with said first and second optical nodes being directly optically connected via their respective line interfaces, and said third and fourth optical nodes being directly optically connected via their respective line interfaces; and at least said second and third optical nodes including an optical line terminal having an all-optical pass-through interface including a plurality of pass-through optical ports, with a selected pass-through optical port in the optical line terminal of said second optical node being directly optically connected to a selected pass-through optical port in the optical line terminal of said third optical node to form an optical path from said first optical node to said fourth optical node.
 5. The wavelength division multiplexed optical communication system of claim 4, further comprising: fifth and sixth optical nodes, each having a line interface, with said fifth and sixth optical nodes being directly optically connected via their respective line interfaces, and at least said fifth optical node including an optical line terminal having an all-optical pass-through interface including a plurality of pass-through optical ports, with a selected pass-through optical port in the optical line terminal of said second optical node being directly optically connected to a selected pass-through optical port in the optical line terminal of said fifth optical node to form an optical path from said first optical node to said sixth optical node.
 6. A wavelength division multiplexed optical communication system comprising: a plurality of optical line terminals, each having an all-optical pass-through interface including a plurality of all-optical pass-through ports, and each also including a plurality of local optical ports; and connecting means for selectively optically connecting at least one of the all-optical pass-through ports of one optical line terminal to at least one of the all-optical pass-through ports of another optical line terminal, and for selectively optically connecting at least one local optical port of one optical line terminal to at least one local optical port of another optical line terminal.
 7. A wavelength division multiplexed optical communication system comprising: a plurality of optical line terminals, each having an all-optical pass-through interface including a plurality of all-optical pass-through ports, and each also including a plurality of local optical ports; and connecting means for selectively optically connecting at least one of the all-optical pass-through ports of one optical line terminal to at least one of the all-optical pass-through ports of another optical line terminal, and for selectively optically connecting at least one local optical port of one optical line terminal to client equipment.
 8. The wavelength division multiplexed optical communication system of claim 7, wherein the client equipment comprises any one of wavelength division multiplexed equipment, SONET equipment, add/drop multiplexers, cross-connect switches, internet protocol routers and asynchronous transfer mode switches.
 9. A wavelength division multiplexed optical line terminal comprising: a first optical demultiplexer which demultiplexes N optical wavelengths of a wavelength division multiplexed optical signal into M optical bands, where N and M are each integers and N>M; and at least a second optical demultiplexer which demultiplexes one of the M optical bands into x individual wavelengths, where x>1.
 10. The wavelength division multiplxed optical line terminal of claim 9, wherein the optical line terminal is of modular construction, and said first optical demultipelxer and said at least a second optical demultiplexer are situated on separate modular cards.
 11. The wavelength division multiplexed optical line terminal of claim 9, further comprising: output means which outputs the x individual wavelengths at x local optical ports, and outputs the M optical bands, other than the one of the M optical bands demultiplexed by said second optical demultiplexer, at respective pass-through optical ports.
 12. The wavelength division multiplexed optical line terminal of claim 11, wherein the optical line terminal is of modular construction, and said first optical demultiplxer, said at least a second demultiplexer, the X local optical ports and the respective pass-through optical ports are each situated on separate modular cards.
 13. A method of upgrading two separate in service wavelength division multiplexed point-to-point optical communication systems, the first system including first and second optical nodes and the second system including third and fourth optical nodes, each such node having a line interface, with said first and second optical nodes being directly optically connected via their respective line interfaces and, said third and fourth optical nodes being directly optically connected via their respective line interfaces, with at least said second and third optical nodes including an optical line terminal having an all-optical pass-through interface including a plurality of pass-through optical ports, said method comprising the step of: selectively connecting a first pass-through optical port in the optical line terminal of the second optical node to a first pass-through optical port in the optical line terminal of the third optical node to form an optical path from the first optical node to the fourth optical node.
 14. The method of claim 13 including a third separate in service wavelength division multiplexed point-to-point optical communication system, the third system including fifth and sixth optical nodes each having a line interface, with said fifth and sixth optical nodes being directly optically connected via their respective line interfaces, and at least said fifth optical node including an optical line terminal having an all-optical pass-through interface including a plurality of pass-through optical ports, further comprising the step of: selectively connecting a second pass-through optical port in the optical line terminal of the second optical node to a pass-through optical port in the optical line terminal of the fifth optical node to form an optical path from said first optical node to said sixth optical node.
 15. A method of upgrading two separate in service wavelength division multiplexed optical network communication systems to form a merged optical network communication system, the first system including first and second optical nodes and the second system including third and fourth optical nodes, each such node having a line interface, with said first and second optical nodes being optically connected via their respective line interfaces through a first optical network and, said third and fourth optical nodes being optically connected via their respective line interfaces through a second optical network, with at least said second and third optical nodes including an optical line terminal having an all optical pass-through interface including a plurality of pass-through optical ports, said method comprising the step of: selectively connecting a first pass-through optical port in the optical line terminal of the second optical node to a first pass-through optical port in the optical line terminal of the third optical node to form an optical path from the first optical node of the first optical network communication system to the fourth optical node of the second optical network communication system.
 16. The method of claim 15 including a third separate in service wavelength division multiplexed optical network communication system the third system including fifth and sixth optical nodes each having a line interface, with said fifth and sixth optical nodes being optically connected via their respective line interfaces through a third optical network, and at least said fifth optical node including an optical line terminal having an all-optical pass-through interface including a plurality of pass-through optical ports, further comprising the step of: selectively connecting a second pass-through optical port in the optical line terminal of the second optical node to a pass-through optical port in the optical line terminal of the fifth optical node to form an optical path from the first optical node of the first optical network communication system to the sixth optical node of the third optical network communication system.
 17. A method of upgrading an in service wavelength division multiplexed optical communication system including a plurality of optical line terminals, each having an all-optical pass-through interface including a plurality of all-optical pass-through ports, and each also including a plurality of local optical ports, said method comprising the steps of: selectively connecting at least one local optical port of one optical line terminal to client equipment; and selectively connecting at least one of the all-optical pass-through ports of one optical line terminal to at least one of the all-optical pass-through ports of another optical line terminal.
 18. A method of upgrading an in service wavelength division multiplexed (WDM) optical communication system including first and second separate WDM networks, with the first WDM network having optically connected first and second optical nodes via respective line interfaces, and the second WDM network having optically connected third and fourth optical nodes via respective line interfaces, with at least the second and third optical nodes including an optical line terminal having an all-optical pass-through interface including a plurality of pass-through optical ports, said method comprising the step of: connecting the first WDM network to the second WDM network by selectively connecting a first pass-through optical port in the optical line terminal of the second optical node of the first WDM network to a first pass-through optical port in the optical line terminal of the third optical node of the second WDM network to form an optical path from the first optical node of the first WDM network to the fourth optical node of the second WDM network.
 19. The method of claim 18 including a third separate WDM network, the third WDM network including fifth and sixth optical nodes each having a line interface, with said fifth and sixth optical nodes being directly optically connected via their respective line interfaces, and at least said fifth optical node including an optical line terminal having an all-optical pass-through interface including a plurality of pass-through optical ports, further comprising the step of: connecting the first WDM network to the third WDM network by selectively connecting a second pass-through optical port in the optical terminal of the second optical node of the first WDM network to a pass-through port in the optical line terminal of the fifth optical node in the third WDM network to form an optical path from the first optical node of the first WDM network to the sixth optical node of the third WDM network.
 20. A wavelength division multiplexed optical communication system comprising: at least first and second optical line terminals, each having an all-optical pass-through interface including a plurality of all-optical pass-through ports, and each also including a plurality of local optical ports, with at least one all-optical pass-through port of said first optical line terminal being connected to a corresponding all-optical pass-through port of said second optical line terminal, and each further including at least one 1×2 switch having an input terminal for receiving at least one input wavelength, a first output terminal connected to said at least one all optical pass-through port and a second output terminal connected to one of said plurality of local port.
 21. The wavelength division multiplexed optical communications system of claim 20, further comprising: a controller for providing a control signal for concurrently controlling the one 1×2 switch in each of said first and second optical line terminals such that when the control signal is in a first state the input terminal is switched to the first output terminal for providing the at least one input wavelength to said at least one all optical pass-through port, and when the control signal is in a second state the input terminal is switched to the second output terminal for providing the at least one input wavelength to the one of said plurality of local ports.
 22. A wavelength division multiplexed optical communication system comprising: at least first and second optical line terminals, each having an all-optical pass-through interface including a plurality of all-optical pass-through ports, and each also including a plurality of local optical ports, with at least one all-optical pass-through port of said first optical line terminal being connected to a corresponding all-optical pass-through port of said second optical line terminal, and each further including at least one lxN switch having an input terminal for receiving at least one input wavelength, at least one output terminal connected to a respective one of said optical pass-through ports and remaining output terminals being connected to respective ones of said plurality of local ports.
 23. The wavelength division multiplexed optical communication system of claim 22, further comprising: a controller for providing a control signal having at least N states for concurrently controlling the at least one 1×N switch in each of said first and second optical line terminals such that when the control signal is in one state the input terminal is switched to the at least one output terminal for providing the at least one input wavelength to said at least one all optical pass-through port. 